[SciPy-user] Getting coordinates of a level (contour) curve

[John Hunter]

On Tue, Aug 12, 2008 at 9:36 AM, Rob Clewley <rob.clewley at gmail.com> wrote:
>> Yes, you are right. But what if I have a mixture of gaussians, or any
>> other 2D probability density function?
>
> Indeed. Isn't the question about how to extract the data points for
> the curve from the 'contour' object in matplotlib, in the general
> case? Unfortunately I don't have the answer to that, but maybe
> introspection of the object would lead to an answer. From the API doc
> I see a mysterious attribute called 'level'.

The mpl contour function returns a matplotlib.contour.ContourSet
instance which has an attribute "level" array of levels that the
contours are drawn on

  In [57]: CS = plt.contour(X, Y, Z)

  In [58]: CS.levels
  Out[58]: array([-1. , -0.5,  0. ,  0.5,  1. ,  1.5])

It also has an equal length list of line collections
(matplotlib.collections.LineCollection) which you can use to extract
the x, y vertices of the contour lines at a given level.  For a single
level, the line collection may contain one or more independent lines
in the collections.  Here is some example code to get you started:

In [59]: level0 = CS.levels[0]

In [60]: print level0
-1.0

In [61]: c0 = CS.collections[0]

In [62]: paths = c0.get_paths()

In [63]: len(paths)
Out[63]: 1

In [64]: path0 = paths[0]

In [65]: xy = path0.vertices

In [66]: xy.shape
Out[66]: (237, 2)

In [67]: xy[:10,]
Out[67]:
array([[-0.15      , -0.95150169],
       [-0.15877627, -0.95      ],
       [-0.175     , -0.94720234],
       [-0.2       , -0.94221229],
       [-0.225     , -0.93652781],
       [-0.25      , -0.93013814],
       [-0.26810676, -0.925     ],
       [-0.275     , -0.9230207 ],
       [-0.3       , -0.91514134],
       [-0.325     , -0.90651218]])

In [68]: